This invention relates to the fabrication of glass sheet prepared from glasses exhibiting high strain points for use in applications requiring glass sheet of precision flatness and exhibiting defect-free surfaces. One particular application for this invention is the production of glass substrates for use in liquid crystal display (LCD) devices. The fabrication of such devices has been extensively discussed and the details of their operation explained in both the patent and scientific literature. U.S. Pat. No. 5,116,787 (Dumbaugh, Jr.) is illustrative of such literature.
As is explained in that patent, glass has been chosen as a substrate in liquid crystal display devices for at least three basic reasons: first, it is transparent; second, it can withstand the chemical and physical (including thermal) conditions to which it is exposed during display processing; and, third, it can be produced at reasonable cost in thin sheets with precisely controlled dimensions. Liquid crystal displays are passive displays which are dependent upon external sources of light for illumination. They are fabricated as segmented displays or in one of two basic matrix configurations. The substrate needs of the two matrix types differ. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second type is extrinsic or active matrix addressed in which an array of diodes, metal insulator-metal devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both designs, however, two sheets of glass comprise the structure of the display.
As is discussed in that patent, intrinsically addressed liquid crystal displays are fabricated at relatively low temperatures, viz., .ltoreq.350.degree. C. Accordingly, soda lime silicate glass sheet having a silica barrier layer thereon to prevent migration of Na.sup.+ ions has been employed extensively as substrates therefor. A high performance version of intrinsically addressed liquid crystal displays, denoted the super twisted nematic, added to the substrate the requirement of extremely precise flatness. That requirement has mandated that the soda lime silicate glasses utilized in those displays be polished. Alternatively, Corning Code 7059 glass, a barium boroaluminosilicate glass marketed by Corning Incorporated, Corning, N. Y., which is precision formed into sheet requiring no surface polishing using the downdraw fusion pipe, such as is disclosed in U.S. Pat. Nos. 3,338,696 (Dockerty) and 3,682,609 (Dockerty), has been employed.
Extrinsically addressed liquid crystal displays can be subdivided into two categories: the first founded in metal-insulator-metal or amorphous silicon (a-Si) devices; and the second founded in polycrystalline silicon (poly-Si) devices. Because devices formed from poly-Si are processed at substantially higher temperatures than those employed with a-Si thin film transistors, glass compositions having strain points higher than soda lime silicate glasses and Corning 7059 glass (strain point of 593.degree. C.) have been demanded in order to prevent thermal deformation of the sheet during processing and to prevent stress buildup. The practical solution to the problem was to formulate glass compositions exhibiting high strain points such that dimensional change during processing at temperatures above about 600.degree. C. will be minimized. The use of glasses exhibiting strain points in excess of 600.degree. C. can eliminate the need for careful annealing thereof when used in the fabrication of the relatively low temperature a-Si devices. Moreover, contamination of the thin film transistors by alkali metal migrating from the glass surface has been a recognized concern, which concern has resulted in the use of coatings to provide barrier layers on the substrate. Accordingly, recent glass composition research has been directed to the use of alkali metal-free glass compositions for use as substrates. And the great proportion of that research has been directed to the formulation of glasses having strain points higher than 600.degree. C., preferably higher than 625.degree. C., and most preferably higher than 650.degree. C. Furthermore, because of the desire to eliminate the need for surface polishing, glass compositions have been sought which can be precision formed utilizing the above-mentioned downdraw fusion pipe. Glasses satisfying those criteria have been developed in the alkaline earth metal aluminosilicate system which are essentially free of alkali metal oxides and may desirably contain boron oxide. The following disclosures are illustrative of the research for glass compositions suitable for the manufacture of sheet glass for LCD devices.
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) describes glasses consisting essentially, expressed in terms of cation percent, of 52-58% SiO.sub.2, 12.5-18% Al.sub.2 O.sub.3, 20-23% B.sub.2 O.sub.3, 0-4% MgO, 0-6% CaO, 0-6% SrO, 1-9% BaO, 8-12% MgO+CaO+SrO+BaO, 0-3% ZnO, and 0-1% fining agents. The glasses exhibit strain points in excess of 625.degree. C., a linear coefficient of thermal expansion (25.degree.-300.degree. C.) of 20-60.times.10.sup.-7 /.degree.C., and a weight loss in 5% HCl at 95.degree. C. not exceeding 10 mg/cm.sup.2 after 24 hours.
U.S. Pat. No. 5,116,787 (Dumbaugh, Jr.) reports glass compositions essentially free of MgO and alkali metal oxides and consisting essentially, expressed in terms of mole percent, of 63-68% SiO.sub.2, 7.5-11% Al.sub.2 O.sub.3, 9.5-16% CaO, 0-5% SrO, 4.5-10% BaO, 14-26% CaO+SrO+BaO, and 1-7% B.sub.2 O.sub.3. The glasses demonstrate strain points of at least 655.degree. C., linear coefficients of thermal expansion (25.degree.-300.degree. C.) between about 35-50.times.10.sup.-7 /.degree.C., and excellent resistance to attack by 5% HCl at 95.degree. C. after 24 hours.
U.S. Pat. No. 5,116,788 (Dumbaugh, Jr.) records glass compositions essentially free from alkali metal oxides and consisting essentially, expressed in terms of mole percent, of 60-65% SiO.sub.2, 8-10% Al.sub.2 O.sub.3, 1-4% B.sub.2 O.sub.3, 11-24% CaO, 0-4% MgO, 0-12% SrO, 0-9% BaO, 23-28% CaO+MgO+SrO+BaO. The glasses display strain points in excess of 675.degree. C. and a weight loss in 5% HCl at 95.degree. C. of less than 1 mg/cm.sup.2 after 24 hours.
U.S. Pat. No. 5,116,789 (Dumbaugh, Jr. et al.) is drawn to glass compositions essentially free from MgO and alkali metal oxides and consisting essentially, expressed in terms of mole percent, of 65-75% SiO.sub.2, 6-10% Al.sub.2 O.sub.3, 15-26% SrO, 0-10% CaO and/or BaO, 0-5% B.sub.2 O.sub.3, and 0-12% B.sub.2 O.sub.3 +[CaO and/or BaO]. The glasses exhibit strain points higher than 675.degree. C., linear coefficients of thermal expansion (25.degree.-300.degree. C.) between about 35-65.times.10.sup.-7 /.degree.C., and excellent resistance to attack by 5% HCl at 95.degree. C. after 24 hours.
U.S. Ser. No. 08/8,560, filed Jan. 23, 1993 under the title HIGH LIQUIDUS VISCOSITY GLASSES FOR FLAT PANEL DISPLAYS by W. H. Dumbaugh, Jr. and J. C. Lapp and assigned to the same assignee as the present application, refers to glass compositions essentially free from alkali metal oxides and consisting essentially, expressed in terms of mole percent, of 64-70% SiO.sub.2, 9.5-12% Al.sub.2 O.sub.3, 5-10% B.sub.2 O.sub.3, 0-5% TiO.sub.2, 0-5% Ta.sub.2 O.sub.5 0-5% MgO, 3-13% CaO, 0-5.5% SrO, 2-5.5% BaO, and 10-20% MgO+CaO+SrO+BaO. The glasses demonstrate strain points in excess of 650.degree. C., linear coefficients of thermal expansion (0.degree.-300.degree. C.) between 32-46.times.10.sup.-7 /.degree.C., and a weight loss in 5% Hcl at 95.degree. C. of less than 2 mg/cm.sup.2 after 24 hours.
U.S. Ser. No. 08/61,459, filed May 17, 1993 under the title BARIUM ALUMINOSILICATE GLASSES by W. H. Dumbaugh, Jr. and J. C. Lapp and assigned to the same assignee as the present application, discusses glass compositions essentially free from alkali metal oxides and consisting essentially, expressed in terms of mole percent, of 65-76% SiO.sub.2, 7-11% Al.sub.2 O.sub.3, 12-19% BaO, 0-5% B.sub.2 O.sub.3, 0-5% MgO, 0-10% CaO, 0-10% SrO, 0-15% MgO+CaO+SrO, 0-2.5% ZrO.sub.2 0-3% TiO.sub.2, 0-3% Ta.sub.2 O.sub.5, 0.5-5% ZrO.sub.2 +TiO.sub.2 +Ta.sub.2 O.sub.5. The glasses display strain points higher than 660.degree. C., linear coefficients of thermal expansion (0.degree.-300.degree. C.) between about 45-55.times.10.sup.-7 /.degree.C. and a weight loss in 5% HCl at 95% of less than 0.5 mg/cm.sup.2 after 24 hours.
Although glass sheet formed utilizing the downdraw fusion pipe requires no surface polishing as drawn, subsequent handling and shipping can result in injuring the surface of the sheet. Consequently, various measures have been taken to protect the glass surface from abuse until the sheet is ready for use in a particular application. For example, sheets of paper and other materials have been interleaved with the glass sheets during stacking for shipping to prevent glass-to-glass contact. As can be appreciated, that practice entails a separate step in the overall process with additional costs, and demands a high degree of care not only in the interleaving procedure as the sheets are being stacked for shipment, but also when the sheets are withdrawn from the stack.